The present invention relates to the field of downhole oil, gas, and/or geothermal exploration and more particularly to the field of resistivity tools for tool strings employed in such exploration.
For the past several decades, engineers have worked to develop apparatus and methods to effectively obtain information about downhole formations, especially during the process of drilling. Logging-while-drilling (LWD) refers to a set of processes commonly used by the industry to obtain information about a formation during the drilling process in order to transmit the information from components located downhole on oil and gas drilling strings to the ground's surface. Various sensors and methods have been developed to obtain and transfer formation information to the surface.
Another problem the industry has sought to address has been invasion of the formation by drilling mud. As drilling mud invades the surrounding formation, the resistivity of the drilling mud may mask the resistivity of the native formation. Varying combinations of transmitter-receiver distances have been used in order to explore formations at various depths of investigation, some utilizing multiple signal frequencies.
The prior art contains references to resistivity tools and resistivity logging.
U.S. Pat. No. 6,677,756 to Fanini, et al, which is incorporated by reference for all that it contains, discloses an induction tool for formation resistivity evaluations. The tool provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions.
U.S. Pat. No. 6,359,438 to Bittar, which is incorporated by reference for all that it contains, discloses a resistivity tool for use in an LWD system that includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters are selectively energized, causing current to be induced in the collar of the tool.
U.S. Pat. No. 6,577,129 to Thompson, et al, which is incorporated by reference for all that it contains, discloses an electromagnetic wave propagation resistivity borehole logging system comprising multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies.
U.S. Pat. No. 6,538,447 to Bittar, which is incorporated by reference for all that it contains, discloses a multi-mode resistivity tool for use in a logging-while-drilling system that includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic signals at multiple depths of investigation.
U.S. Pat. No. 7,141,981 to Folbert, et al, which is incorporated by reference for all that it contains, discloses a resistivity logging tool suitable for downhole use that includes a transmitter, and two spaced apart receivers. The resistivities at the two receivers are corrected based on measuring the responses of the receivers to a calibration signal.
U.S. Pat. No. 6,218,842 to Bittar, et al, which is incorporated by reference for all that it contains, discloses a resistivity tool for use in LWD systems that includes an asymmetric transmitter design with multiple transmitters capable of generating EM signals at multiple frequencies.
U.S. Pat. No. 6,810,331 to Bittar, et al, which is incorporated by reference for all that it contains, discloses a system and related method for obtaining electromagnetic resistivity logs of a formation having fixed depths of investigation that differ from the actual depth of investigation for the tool.
U.S. Pat. No. 6,777,940 to Macune, which is incorporated by reference for all that it contains, discloses a well logging apparatus and methods for determining formation resistivity at multiple (>3) depth of investigation. At least one transmitter antenna and at least two receiver antennas are mounted in a logging tool housing, on substantially a common axis. The antennas are untuned coils of wire. The apparatus minimizes the number of antennas, electronics, complexity, required power, and measurement time required to determine resistivity at multiple depth of investigation.